This invention relates in general to bearings and more particularly to a bearing assembly that is capable of monitoring angular velocity and a process for assembling the same.
The automotive industry has turned to antilock brake systems to improve control over vehicles when the brakes of such vehicles are applied, particularly during emergency stops and braking on slippery surfaces. An antilock brake system must monitor the speed of each wheel to detect whenever a wheel locks up and begins to skid, so that the system may relax the braking effort applied to that wheel, at least to the point that the wheel again begins to rotate. With all wheels rotating, the driver maintains better control over the vehicle.
Typically, the hub for each road wheel carries a target or exciter wheel which rotates with the road wheel at the same angular velocity. As the target wheel turns, its periphery moves past a sensor which is generally fixed in position on the vehicle. The moving target wheel causes the sensor to generate a signal which may appear as a sine wave or simply as pulses, but in any event is characterized by a frequency, and that signal along with corresponding signals from all of the other wheels are processed by a controller. Thus, the controller will sense when any one of the wheels locks up or rotates at a diminished velocity. On most automotive vehicles of current manufacture, the target wheels exist on the exterior surfaces of the wheel hubs where they are exposed to contaminants such as dirt and dust, as well as to corrosive chemicals. These substances may damage the target wheel or lodge in the gap between the target wheel and the sensor and thereby disrupt the sensor.
The bearing assemblies on which the hubs rotate of course have seals for isolating raceways and the rolling elements within them from the contaminants to which the exterior of the hub is subjected. Others have recognized that these seals may likewise be used to protect a target wheel and to isolate the gap that exists between the target wheel and the sensor, so that sand, dirt and other solid objects do not enter the gap. After all, the size and integrity of the gap are quite important, at least where the sensor is the typical variable reluctance type, which is the type most commonly used on antilock braking systems, because the voltage produced by the sensor varies in inverse proportion to the size of the gap. That voltage of course represents the signal delivered to the processor of the antilock braking system, and it must be strong enough to be detected. Some bearing assemblies are assembled without any regard for the size of the gap between the target wheel and sensor, that is to say, the assembly procedure does not include, nor does the bearing assembly provide for an adjustment of the gap. In this type of bearing assembly tolerances may accumulate and render the size of the gap excessive. Other bearing assemblies provide means for adjusting the position of the sensor, but they usually represent an extra part or two and added complication in the assembly procedure.
The present invention resides in a bearing assembly having a target wheel isolated between the seals of the bearings where it is monitored by a sensor. The target wheel has an operative surface which is beveled with respect to the axis of rotation for the bearing and the sensor lies along an axis which is oblique to the bearing axis and perpendicular to the operative surface of the target wheel. The gap between the sensor and the target wheel depends on the axial position of the target wheel, and indeed the target wheel is moved axially over one of the races while a gage measures the distance between the operative surface and a reference surface that is later used to locate the sensor.